The invention relates to a hydraulic control device of the type indicated in the precharacterizing part of claim 1.
In a control device of this type known from practice, e.g. for the lifting cylinder of a stacker truck, the three-way flow controller with its measuring orifice is disposed in the working pressure line and in the direction of flow to the hydraulic motor before the non-return valve. The bypass line bypasses the non-return valve and is moreover connected to the control outlet of the three-way flow controller. The two-way flow controller with its measuring orifice is located in the bypass line. The mentioned flow controllers distinguish themselves by the fact that they keep the amount of pressure fluid directly constant as a function of the flow influence of the proportional magnet of the measuring orifice independently of the load on the hydraulic motor and allow the sensitive change of the speed of the hydraulic motor, again independently of the load by a change of the flow influence of their proportional magnet. Such flow controllers are e.g. shown and described in the leaflet D 7177 "Stromregelventile", January 1985, of Heilmeier & Weinlein, 8 Munich 80. Their expensive and complex construction is disadvantageous in the known control devices, in which two expensive proportional magnets with their measuring orifices are necessary, which must be electrically excitable independently of each other.
A control device was developed with the attempt of getting away from the high constructional expenditure as it is known from the DE-OS No. 32 33 046, FIG. 1. In this control device a two-way flow controller with a measuring orifice being actuated by a single proportional magnet is only provided in the bypass line to the tank. The flow controller influences the amount of pressure fluid flowing from the pressure source to the hydraulic motor or from the hydraulic motor into the tank by discharging in each case an amount of pressure fluid depending on the flow influence on the proportional magnet into the tank. The serious disadvantage ensues from this that the amount of pressure fluid passing through the flow controller is constant to the influence of flow on the proportional magnet, that, however, the amount of pressure fluid flowing to the hydraulic motor is no longer proportional to the influence of flow on the proportional magnet. The pressure source is in such cases of application a pump only running during load upon operation of the hydraulic motor, which is driven by a shunt electric motor, in which the amount conveyed per time unit is changed with increasing counter-pressure. The two-way flow controller cannot compensate for these changes towards the tank, which means that the hydraulic motor operates more slowly with increasing load and gets faster with decreasing load. This is undesired in practice where a sensitive speed control is of importance. Moreover an undesired switching jerk occurs during starting.